Dual-beam interferometers are used for spectral analysis of a gas sample contained in a standardized chamber. A first beam of electromagnetic radiation is projected into the gas to be analyzed in a first chamber and then is projected to the interferometer, which creates an interferogram on a detector. A complementary second beam projected through a second identical but empty chamber is also submitted to the interferometer, which creates an interferogram of it on the detector. Thus any spurious background spectral signal variations derived from the first chamber are combined with those derived from the second, thereby eliminating these variations and leaving only the spectral signal variations derived from the gas.
More recently, it has been suggested to use interferometers to provide, on a detector having a multitude of cells, interferograms of an object area in attempting to locate a source of radiation in the object area that differs only slightly spectrally, and usually not more than slightly in intensity, from the background of the object area. For example, the background may have an emission band of 2.95 - 3.05.mu., while the source has an emission band of 2.90 - 3.05.mu.. The source is expected to be a point-like object being comparable in size to a spatial resolution element of the optical system and appears contrasted against a smoothly varying background which is often spectrally similar to the source. Each cell on the detector may represent an object area of 300 meters radius, while the source, for example, is but 50 meters in radius or approximately 3% of the area. Nevertheless, it is desired to identify that cell corresponding to the object area containing that small source, 3% of the object area, for further processing. According to one conventional technique to effect this, the output from each cell is submitted to a device such as a computer, which takes the Fourier transforms of the interferogram output of each cell to determine the spectrum of the output of each cell. If the analysis shows that a cell has the band 2.90 - 3.05.mu. instead of the expected 2.95-3.05.mu., then the object area corresponding to that cell contains the source sought. However, this is a time-consuming, expensive technique, since each cell in a multicell detector containing perhaps 10.sup.4 to 10.sup.6 cells must be processed. It would thus be quite advantageous to be able to select for further processing only those cells most likely to contain information including the sought spectral characteristic. Unfortunately, in such applications the background is typically a portion of the earth's surface or other area not accurately enough reproducible for use as a complementary image to enable the background to be offset and eliminated in the manner suggested previously with dual-beam interferometer gas analyzers.